In reflective electron beam lithography, a dynamic pattern generator (DPG) is utilized to deliver exposure patterns. The DPG includes an array of mirror cells. In a process for patterning a wafer, some mirror cells in the array may absorb electrons, while the other mirror cells may reflect electrons back to the wafer. As the wafer size continues to increase, the dimension of the mirror array increases as well. DPGs are required to operate at a relatively high data rate and toggling rate. In patterning a wafer, however, if all mirror cells are toggled at the same time, a transient spike current may incur a significant dynamic IR drop, which may deteriorate the patterning performance.